A new study shows how quantum systems can evolve simultaneously along with two opposing time arrows – both forward and backward in time.
The research, which was published in the most recent issue of Communications Physics, calls for a rethinking of how the flow of time is understood and represented in situations where quantum laws are important.
Since the dawn of time, philosophers and scientists alike have been debating the question of whether or not there is a such a thing as time. But our experience in the classical world seems to dispel any question that time exists and continues. An arrow of time may be traced in nature by the fact that processes progress from a state of less disorder to a one of more disorder. ‘Entropy’ is a physical quantity that measures the amount of disorder in a system, and describes this in physics terms.
Professor Giulia Rubino, the lead author of the study and a researcher at the Quantum Engineering Technology Labs (QET Labs) at the University of Bristol, says:
“If a phenomenon produces a large amount of entropy, observing its time-reversal is so improbable as to become essentially impossible. However, when the entropy produced is small enough, there is a non-negligible probability of seeing the time-reversal of a phenomenon occur naturally.
“We can take the sequence of things we do in our morning routine as an example. If we were shown our toothpaste moving from the toothbrush back into its tube, we would be in no doubt it was a rewinded recording of our day. However, if we squeezed the tube gently so only a small part of the toothpaste came out, it would not be so unlikely to observe it re-entering the tube, sucked in by the tube’s decompression.”
The study’s authors, led by Professor Caslav Brukner of the University of Vienna and the IQOQI-Vienna, applied this concept to the quantum realm, one of whose quirks is the principle of quantum superposition, which states that if two states of a quantum system are both possible, the system can be in both states at the same time.
“Extending this principle to time’s arrows, it results that quantum systems evolving in one or the other temporal direction (the toothpaste coming out of or going back into the tube), can also find themselves evolving simultaneously along both temporal directions.
“Although this idea seems rather nonsensical when applied to our day-to-day experience, at its most fundamental level, the laws of the universe are based on quantum-mechanical principles. This begs the question of why we never encounter these superpositions of time flows in nature,” says Dr Rubino.
“In our work, we quantified the entropy produced by a system evolving in quantum superposition of processes with opposite time arrows,” says co-author Dr. Gonzalo Manzano of the University of the Balearic Islands, adding, “we found this most often results in projecting the system onto a well-defined time’s direction, corresponding to the most likely process of the two. And yet, when small amounts of entropy are involved (for instance, when there is so little toothpaste spilled that one could see it being reabsorbed into the tube), then one can physically observe the consequences of the system having evolved along the forward and backward temporal directions at the same time.”
The work has practical implications in quantum thermodynamics, in addition to the fundamental fact that time itself may not be well-defined. Placing a quantum system in a superposition of alternate time arrows could help thermal machines and freezers work better.
Dr Rubino concludes: “Although time is often treated as a continuously increasing parameter, our study shows the laws governing its flow in quantum mechanical contexts are much more complex. This may suggest that we need to rethink the way we represent this quantity in all those contexts where quantum laws play a crucial role.”
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